FIG. 1 is a perspective view depicting a conventional Indium-Gallium-Nitride (InGaN) multi-quantum-well (MQW) structure laser diode 50, which is exemplary of one type of laser diode. Referring to the lower portion of FIG. 1, laser diode 50 includes an n-doped GaN layer 62 formed on a substrate (e.g., SiO2) 60. An n-electrode 64 is formed on the right-side upper surface side of n-doped GaN layer 62. On the left side of n-doped GaN layer 62 is a stack respectively including an n-doped Aluminum-Gallium-Nitride (AlGaN) cladding layer 66, an n-doped GaN separated confinement hetero (SCH) layer 68, a quantum well region 70 comprising multiple InGaN/GaN layers, a p-doped AlGaN barrier layer 72, an 9-doped GaN SCH layer 74, a p-doped AlGaN cladding layer 76, a p-doped GaN layer 78, and a p-electrode 80. During operation, a suitable voltage potential is applied to n-electrode 64 and p-electrode 80, causing electrons and holes to combine in quantum well region 70 in a manner such that a highly coherent (in this case blue) laser beam LB is emitted from a point 51 located on a face 52 of laser diode 50. Additional details regarding the physics involved in generation of laser beam LB by laser diode 50 are beyond the scope of the present invention, and are therefore omitted for brevity.
Laser diodes, such as AlGaN laser diode 50, are used in many applications, such as in the DVD read head of an MP3 player and in fiber optic switches. Such laser diodes are typically mass-produced, and good manufacturers try to make the laser diodes as consistent as possible, but there are deviations in performance (e.g., in the direction, intensity and divergence of the emitted laser beam). In some applications these deviations may not be important. However, certain applications are very sensitive to the divergence and the pointing direction of the laser beam, and it is important to verify laser performance to assure proper operation of the host device. For example, in an MP3 disk player, a laser diode is mounted on a silicon chip that includes control circuitry for the DVD read head. During the mounting process, the laser diode must be mounted properly and the far-field pattern of the laser beam identified so that the optics of the system are aligned to the correct coordinates to be at the center of the beam pattern. Because no adjustments to laser diode are possible once glued in place, the characteristics of laser diode must be determined before this mounting process takes place, or if at the time of attachment, then before the glue is hardened.
A conventional system used to measure the far-field pattern of laser diodes for these sensitive applications uses a video system to transduce data from the laser's light emission far-field pattern to characterize the angular distribution of the laser energy. The desired information about a laser diode pattern are the parameters describing it as a two-dimensional Gaussian pattern, including pointing angles in the X-axis and Y-axis directions plus width angles, such as Full Width at Half Maximum (FWHM). The coordinate system of these parameters has as its origin the emission point of the laser (e.g., emission point 51; see FIG. 1), and physical datums defining the axes angularly are the planar surfaces of the laser device itself (i.e., face 52 of laser diode 50, the solid state chip on which the laser device is fabricated, or another structure into which the laser device is assembled, such as the typical “TO Can” package). Part of the necessary measurement process involves determining the position of the measurement instrument relative to these laser device axes. In other words, one must “pick up” the datum surface of the laser device, as well as the radiation pattern generated by the laser device in order to consistently repeat the testing process for each laser diode.
Several conventional instruments are commercially available for characterizing the angular distribution of laser energy generated by a laser device. One class of such conventional instruments performs this function using a charge-coupled device (CCD) video camera to sense the illumination pattern of laser beams, and to transduce the pattern to a digitized file for detailed analysis of the illumination pattern geometry. This technique has been used for some time to sense the illumination pattern of gas lasers, which generate light that is essentially collimated. Laser diodes, on the other hand, are essentially point sources, so the emitted light is highly divergent (i.e., the geometry of the light pattern scales linearly with distance from the source). Accordingly, laser diodes are characterized by the emitted light intensity versus angle. Therefore, for laser diode characterization, precise measurement of the source point location is necessary, as well as the orientation of the device in angles “pitch” and “roll” (angles θX and θY).
Laser diode characterization requires tooling that determines the exact position of the light-emitting device in 6 dimensions: X, Y, Z and θX, θY, and θZ (although the precision requirement for θZ is low). The positioning of the laser diode may be performed by active alignment or passive alignment. Active alignment is performed using a system that provides a view of the laser and mechanical means to adjust the position of the laser diode to an exact reference position before testing. Passive alignment involves mounting hardware that mechanically constrains the laser diode position with variability that is small in comparison to the desired resolution of the laser far-field pattern measurement system. Typical passive systems use mechanical references only, as with a laser in a “can” (transistor housing) being clamped into a reference mount of precise fit, so the orientation of the housing is determined to some precision.
A typical practice in testing laser diodes is to attach the laser to a precision linear slide device, and affix three instruments parallel to each other and bearing on three “parking positions” for the laser diode under test. The three instruments are a microscope, an autocollimator, and a far-field camera. The laser diode to be tested is adjusted in X, Y and Z directions at the microscope, then translated a measured amount via the linear slide to the position of the autocollimator where its orientation is adjusted in angles θX and θY, then translated another measured amount slide to the third position of the far-field camera where the image of the radiation pattern is detected and measured. Other means are needed to align the three instruments, microscope, autocollimator and far-field camera, so they match precisely in “zero” pointing direction. Typical for this would be a small laser “light pen” device which can be attached to the same slide and aligned to one instrument or the other to the position θX=0 and θY=0, then translated to the other instrument. The second instrument is mechanically realigned in pointing direction to give the same indication θX=0 and θY=0 for that light pen output.
A disadvantage associated with conventional systems for testing laser diodes is that the precise determination of the location of the part under test is problematic. Mechanical, passive alignment systems are not precise enough for orienting the angle of laser diode chips, whose dimensions are typically 0.3 mm square. Optical detection for measuring the X, Y and Z position is typically performed by a microscope, and optical detection for measuring the angles θX and θY is typically performed by an autocollimator, and both of these instruments compete for space with the instrument for laser pattern angle measurement. Combining even two such instruments with a beam-splitter as the element nearest the part to be tested costs working distance, which is generally in very short supply for a microscope objective, and still leaves one function missing. Separating the three instruments by attaching the part to a precision slide mechanism to be tested by all three in sequence is costly in operation time and bulk of the equipment.
Several conventional instruments are available for performing far-field pattern measurements, but these conventional instruments have several deficiencies. First, these conventional instruments typically include hardware and software that is capable of performing many functions that are not related to far-field pattern measurement, thereby making these instruments more expensive than if they were constructed solely for far-field pattern measurement. Second, conventional instruments typically require additional hardware to orient the instrument relative to a laser diode under test in a manner that makes a complete measurement possible. In particular, the conventional instruments lack a capability to measure exact position and orientation of the laser diode in X-axis, Y-axis, Z-axis, and θX and θY orientation angles. When provided in the conventional manner described above, this additional equipment causes the total system to take up significant space. Finally, the process of assembling and operating these conventional instruments along with the needed additional equipment is fraught with complexity of alignment, and is vulnerable to operator error.
What is needed is a low cost, space-efficient, and easily assembled optical test system for determining the far-field pattern of a laser diode that overcomes the problems associated with conventional test systems, described above. In particular, what is needed is an optical test system that facilitates the detection of the emitting point location and far-field pattern pointing direction of a laser diode chip relative to a mechanical datum on the laser diode package. The angles of the laser pattern centerline and of the mechanical datum of the laser must be either constrained or measured to accuracy of about 0.1 degree at the same time.